On the origin of exponential galaxy discs

Abstract

One of the most important unresolved issues for galaxy formation theory is to understand the origin of exponential galaxy discs. We use a disc galaxy evolution model to investigate whether galaxies with exponential surface brightness profiles can be produced in a cosmologically motivated framework for disc galaxy formation. Our model follows the accretion, cooling and ejection of baryonic mass, as a function of radius, inside growing dark matter haloes. The surface density profile of the disc is determined by detailed angular momentum conservation, starting from the distribution of specific angular momentum as found in cosmological simulations. Exponential and quasi-exponential discs can be produced by our model through a combination of supernova-driven galactic outflows (which preferentially remove low angular momentum material), intrinsic variation in the angular momentum distribution of the halo gas and the inefficiency of star formation at large radii. We use observations from the Sloan Digital Sky Survey (SDSS) New York University Value Added Catalog (NYU-VAGC) to show that the median Sérsic index of late-type galaxies is a strong function of stellar mass. For blue galaxies, low-mass galaxies have n≃ 1.3, while high-mass galaxies have n≃ 4, with a transition mass of Mstar≃ 2.5 × 1010 M⊙. Our model with energy-driven outflows correctly reproduces this trend, whereas our models with momentum-driven outflows and no outflows overpredict the Sérsic indices in low-mass galaxies. We show that the observed fraction of ‘bulge-less’ exponential galaxies is a strong function of stellar mass. For Milky Way mass galaxies (Vrot≃ 220 km s−1, Mstar≃ 1011 M⊙), less than 0.1 per cent of blue galaxies are bulge-less, whereas for M33 mass galaxies (Vrot≃ 120 km s−1, Mstar≃ 1010 M⊙) bulge-less and quasi-bulge-less galaxies are more common, with 45 per cent of blue galaxies having the Sérsic index n < 1.5. These results suggest that the difficulty of hierarchical formation models to produce bulge-less Milky Way mass galaxies is, in fact, not a problem. However, the problem of producing M33-like galaxies remains, and will provide a key test for hierarchical galaxy formation models, and feedback models in particular.